The present invention relates to a MIS semi-conductor integrated circuit containing MISFETs as circuit elements and, more particularly, to a MIS semiconductor integrated circuit containing MISFETs of which the effective channel length is approximately 1 .mu.m or less.
With the substantial progress recently made in the development of semiconductor integrated circuits containing MOSFETs as one type of MISFETs, integrated circuits containing several tens or hundreds of MOSFETs with an effective channel length of about 10 .mu.m per chip were realized in the later half of 1960. The advanced micro-fabrication technique has further improved the integration density of the integrated circuits, thus allowing for the realization of VLSIs containing several hundred thousand elements, with an effective channel length of about 1.5 .mu.m per chip.
Submicron semiconductor integrated circuits having submicron MOSFETs with an effective channel length of 1 .mu.m or less may be realized in near future.
In the conventional MIS semiconductor integrated circuit, the internal circuit is directly driven by a power source voltage which is externally supplied. The value of the power source voltage decreases with the reduction of the effective channel length of the MISFETs (MOSFETs), making up the internal circuit. A 5 V power source voltage is used in driving the internal circuit of semiconductor integrated circuits containing MOSFETs with an effective channel length of, for example, 1.5 .mu.m.
Reduction of the effective channel length of the MOSFET increases the electric field thereof, if the power voltage is fixed. An increase in the electric field of the MOSFET adversely influences its elementary characteristics in the following ways.
1. Hot electrons or hot halls are generated, due to the impact of ionization. PA1 2. The substrate current is increased. PA1 3. The punch-through withstand capacity is reduced. PA1 4. PN contacts containing sources and drains are broken down. PA1 5. The threshold voltage changes over time, due to the trapping of hot carriers within the gate insulation film.
To avoid the above disadvantages, strict requirements are applied in setting the value of the externally applied power source voltage.
In the future, the submicron semiconductor integrated circuit will require a power source with a voltage level lower than that of the 5 V power source voltage, which is currently used as a standard power source, to avoid the above problems (1-5). The aging of the threshold voltage of the MOSFET, which is due to the trapping of hot carriers (See 5, above), may markedly deteriorate the speed performance of the submicron semiconductor integrated circuit, or may cause the imperfect operation of the submicron semiconductor integrated circuit.
FIG. 1 shows a cross-section illustrating the structure of a general enhancement type MOSFET. In the figure, reference numeral 1 designates a P type silicon semiconductor substrate. An n.sup.+ type source region 2 and an n.sup.+ type drain region 3 are diffused into the surface region of the substrate 1. A gate electrode 6 made of polysilicon, for example, is formed above a channel region 4 located between the source and drain regions 2, 3, while overlaying a gate insulating film 5. In FIG. 1, "Leff" indicates the effective channel length of the MOSFET.
FIG. 2 illustrates an energy band of the MOSFET shown in FIG. 1. A potential barrier .phi.e for electrons, which is formed at the boundary between the substrate 1 and the gate insulating film 5, is 3.1 eV; and a potential barrier .phi.h for holes is approximately 3.8 eV. Let us assume that the geometry of the MOSFET is further reduced and the effective channel length (Leff) is reduced to 1 .mu.m or less. Under such geometrical conditions, if the power source voltage is kept at 5 V, the probability that hot electrons and holes generated by impact ionization will exceed the above potential barriers .phi.e, .phi.h and enter the gate insulating film 5 is high. These electrons and holes entering the gate insulating film 5, are then trapped by the same film, so that the aging of the threshold voltage is great. In assembling many and various types of semiconductor integrated circuits into a system, it is desirable, for size and cost-reduction purposes, that the semiconductor integrated circuits be operable by a single and common power source. It is also preferable that the submicron semiconductor integrated circuit be operable by a standard power source voltage of 5 V, since it is frequently used in connection with the TTL circuits. In semiconductor integrated circuits of the type in which the internal circuit is directly driven by an external power source, however, use of the 5 V power source voltage is accompanied by a deterioration of the circuits characteristic, as mentioned above, possibly making the elements defective.
Further, in conventional semiconductor integrated circuits, such factors as operating speed and power dissipation depend largely on the externally applied power source voltage. This makes the circuit design complex, so that the semiconductor integrated circuits are not easy to handle in their systems applications.
Additionally, the conventional semiconductor integrated circuit must be operated by using a high precision power source. If it is driven by a poor precision power source, an overvoltage application might cause the internal MOSFETs to be deteriorated, degrading their reliability. Moreover, when a spike voltage or noise appears on the power line, the internal MOSFETs are deteriorated or erroneously operated.